I know this one - my mother 91 has appetite of a bird and had to put on weight after surgery. GP did refer (eventually) to a dietician so perhaps you could try that. Little meals spread through the day - cake, biscuits, choc, full fat milk, etc. What does your mil like to eat? My mum doesn't really like meat with veg & pots but does like puddings and when she is feeling off colour will eat custard, rice pudding, yoghurt etc. so if your mil is like that then leave lots of things like that for her. M&S do some little meals (not a great variety) cottage pie and the like. Mum sometimes would only eat a half of one with a little veg - M&S do tiny packets of beans or peas. She did put on weight doing this little and often thing so perhaps it would work for you. I got quite stressed about her not eating which didn't really help anyone but we got there in the end.
Her weight now goes up and down a bit as, if she has a cold or anything it kills any small appetite she has. One more thing, if you go to a lot of trouble, cooking and freezing meals, and then she doesn't eat them, she will feel bad about it and you will tire yourself out. Don't feel bad about buying stuff (I used to cook, freeze, stress!)
It may take a few months but it will work.